In applications with non-volatile memory units, charge pumps are typically used to provide a stable high-voltage level that is required to enable writing- or erasing-operations of the memory unit. For supplying a stable and constant voltage level, such charge pumps typically require regulation. In the absence of effective regulation, the charge pump output voltage may vary depending upon environmental conditions, electric load and the processing parameters under which the charge pump was fabricated. Among a variety of regulating circuits, capacitive or resistive dividers are widely used, by way of which a high voltage level on the output of the charge pump can be sensed for instance even in the absence of any static current load. Respective dividers are operable to divide the high voltage to a level which can be processed by a regulation or feedback loop that is typically operable to compare the downscaled voltage with a reference voltage.
Such feedback loops typically comprise a comparator having a first input connected to a node of the divider and having a second input connected to a reference voltage. The output of the comparator is typically coupled to or connected with the input of a charge pump. In the event, that the feedback loop detects a variation of the output voltage level of the charge pump, a respective and compensating control signal will be generated by the comparator. Such a regulation scheme is typically referred to as a continuous regulation, where the feedback loop changes the charge pump power continuously to achieve a desired output voltage level.
There also exists an ON/OFF regulation scheme, wherein the charge pump is regularly and alternately switched on and off. This regular switching of the charge pump inevitably leads to a rippled structure of the charge pump output voltage and also causes some current pulses on the power supply, which in turn causes noise on the supply voltage. These phenomena, the output voltage ripples and such current pulses are usually increasing with the charge pump power.
Up to a certain limit, the power of the charge pump is usually increasing with its operating frequency. However, the charge pump power efficiency is generally decreasing with an increase of the charge pump driving frequency. From this point of view, it is therefore desirable to drive the charge pump at a lowest possible frequency, which is just sufficient for the charge pump to generate a desired output voltage level. However, such an optimal frequency generally depends on the actual operation conditions, like charge pump output load, input supply voltage as well as on external conditions, such like temperature.
The U.S. Pat. No. 6,300,839 B1 describes an approach to adapt the charge pump frequency to varying operation conditions. There, a plurality of differential amplifiers is implemented, wherein each differential amplifier receives a different reference voltage as well as a common input voltage derived from the pumped voltage. A predetermined logic signal output by the differential amplifiers modifies, i.e. reduces, an original frequency of the oscillator. In this manner, the charge pump system may quickly compensate for any overshoot in the pumped voltage in a manner directly correlated to the magnitude of the pumped voltage.
If no differential amplifier outputs the predetermined logic signal, then the oscillator generates the original frequency. In this manner, the charge pump system also compensates for any undershoot in the pumped voltage by providing the fastest frequency. However, such a solution requires implementation of a series of differential amplifiers, which is rather space wasting and expensive. Moreover, such a solution does not seem to fulfill the demands for a miniaturized design of such regulation circuits.
Therefore, the regulation circuit in combination with the charge pump should just supply an output load which is needed by the specific application driven by the charge pump. This generally implies making use of a given relatively high frequency. However, most of the time, such a maximum output load is only required under rare worst case conditions, for instance at a maximum temperature. During most of the circuit lifetime, such high frequency is not needed but causes a non-optimal power efficiency of the charge pump.